Method of terminating a lamp filament

ABSTRACT

A LAMP FILAMENT IS RELIABLY AND FIRMLY CONNECTED BOTH ELECTRICALLY AND MECHANICALLY TO A LEAD WIRE OR MOUNTING POST BY A BODY OF METALLIC MATERIAL INTIMATELY ENTRAPPING THE FILAMENT AND METALLURGICALLY BONDED TO THE LEAD WIRE. IN ORDER TO TERMINATE THE FILAMENT, THE FILAMENT IS PLACED ON THE LEAD WIRE AND OVERLAID WITH A BODY OF METAL. THE METAL IS HEATED TO A PLASTIC OR SEMI-PLASTIC AND NON-LIQUID STATE AND IS PRESSED AGAINST THE FILAMENT AND LEAD WIRE SO THAT THE FILAMENT IS ENVELOPED BY THE METAL AND THE METAL IS DIFFUSION BONDED TO THE LEAD WIRE.

Feb. 9,1971 I r I R. E. suvls 3,561,034

METHOD OF TERMI NATING A LAMP FILAMENT Filed Oct. 4, 1968 PRESSURE E FIG. 3

HEAT and PRESSURE I6 IN VENTOR: RICHARD E. SIMS,

By a 0), KOIQhF I Qn I gcwbhbotp, Udvs Affal'fleysf v United States Patent 3,561,084 METHOD OF TERMINATING A LAMP FILAMENT Richard E. Sims, Des Plaines, Ill., assignor to Chicago, Miniature Lamp Works, Chicago, 11]., a corporation of Illinois Filed Oct. 4, 1968, Ser. No. 765,149 Int. Cl. H01j 9/18, 9/36 US. Cl. 29-25.15 8 Claims ABSTRACT OF THE DISCLOSURE A lamp filament is reliably and firmly connected both electrically and mechanically to a lead wire or mounting The present invention relates to the manufacture of incandescent lamps, and more particularly to a new and improved method for terminating a lamp filament.

In the manufacture of incandescent lamps a filament such as a fine tungsten wire is mounted between a pair of lead wires or posts which serve both to support the filament and to complete a path for energization of the filament. Consequently it is of paramount importance that the interconnection between the filament and the lead wires or posts be firm, reliable, and characterized by extremely low electrical resistance.

An important object of the present invention is to provide an improved method of connecting a lamp filament to a lead wire.

Another object is to provide a method capable of producing a firm, strong and reliable mechanical connection between the filament and the lead wire.

A further object is to provide a method for producing a connection between a filament and a lead wire characterized by low resistance.

Another object of the invention is to provide a method for terminating a lamp filament wherein the production of oxide is avoided.

Another object is to provide an improved method for terminating a lamp filament wherein the connection between the filament and the lead wire is small in size and low in profile, and to provide a method particularly suited to the manufacture of miniature lamps such as numeric readout lamps and the like.

In brief, in accordance with the present invention a lamp filament is mounted to a lead wire by sandwiching the filament between the lead wire and a body of metal. The metal is simultaneously heated to a semi-plastic state and pressed against the lead wire. A metallurgical or intermetallic bond is created between the body of metal and the lead wire, and at the same time the filament is entrapped within the body of metal.

The invention together with the above and other objects and advantages may be better understood from the following detailed description of one process embodying the invention, in the course of which reference is made to the drawing wherein:

FIGS. 1-4 are somewhat diagrammatic and fragmentary illustrations of successive operations carried out in a process of terminating a lamp filament in accordance with the present invention.

Having reference now to the drawing, there are illustrated certain of the steps of a process performed in accordance with the present invention for mounting a lamp filament 10 to a lead wire 12 with a body 14 of metal. In general, and referring first to FIG. 1, the filament 10 is placed on the lead wire 12 and the body 14 of metal is positioned to overlie the filament 10- and lead wire 12. The metal body 14 is then moved toward the lead wire 12 to sandwich the filament 10 between the lead wire 12 and the body 14, as indicated in FIG. 2.

In accordance with an important feature of the invention the body 14 is heated to a plastic or semi-plastic state, but not to a liquid state, while being pressed against the filament 10 and the lead wire 12. As a result the filament is intimately surrounded or entrapped in the metal of the body 14 while an intermetallic or metallurgical bond is formed by diffusion at the interface of the body 14 and the lead wire 12. Due to the extensive and firm mechanical contact between the filament 10 and the body 14, and the thermocompression diffusion bond between the body 14 and lead wire 12, a highly reliable and conductive connection is produced.

The method of the present invention is particularly suited to the manufacture of miniature lamps such as numeric readout lamps wherein several filaments such as the filament 10 illustrated in the drawing may be connected between several lead wires or posts 12 and sealed in a vacuum beneath a transparent face plate. In the illustrated embodiment of the invention, the filament 10 is a tungsten wire having a diameter in the neighborhood of 0.3 mil and wound in a spiral having a diameter in the neighborhood of 1 to 2 mils in accordance with known practice. The lead wire or post 12, only the end of which is illustrated, may be of any desired size and configuration, and preferably comprises a conductor adapted to extend from the interior of the finished lamp for completing an electrical connection to the filament 10. In order to permit an intermetallic or metallurgical bond between the lead wire 12 and the body 14 of metal, the lead wire .12 has a surface formed of a metal capable of producing a diffusion bond with the metal of the body 14. l 1

In the preferred embodiment of the invention, the lead wire 12 is formed of a suitable conductive metal such as copper, dumet, or the like, while the body 14 is formed of gold. In order to permit the formation of a diffusion bond between the body 14 and the post 12, the portion of the post .12 to which the body 14 is bonded is provided with a coating of highly pure gold. In addition, in order to prevent diffusion between the body of the conductor 12 and the gold coating, an intermediate layer of nickel orother suitable material may be provided.

The body 14 of metal may be provided in any desired manner. In the illustrated arrangement, a metal wire 16 preferably fabricated of highly pure gold and having a diameter in the neighborhood of 1.5 mils is advanced from a supply such as a spool (not shown) through a tip member 18 having a central passageway 20 (FIG. 1) approximately 2 mils in diameter. The body 14 is initially in the form of a ball or bead having a diameter of about 6 to 8 mils formed at the end of the wire 15.

As illustrated in FIG. 1, in carrying out the method of the present invention the filament 10 is positioned upon the lead wire 12 between the lead wire and the body 14 of metal. In the preferred arrangement the filament 10 is located on the substantially flat end of the lead wire 12, although it should be appreciated that the filament 10 may be attached to other portions of the lead wire 12 if desired. The arrangement shown in the drawing has the advantage that the connection created between the filament and the lead wire is small in size and low in profile and is 3 therefore particularly suited for use in extremely small lamps.

After the body 14, filament 10, and lead wire 12 have been aligned with one another, the body 14 is moved toward the filament and lead wire 12 as illustrated in FIG. 2. In the preferred method, the body 14 is pressed toward the lead wire 12 before heat is applied to the body 14. This operation serves to compress or crush the spiral wound filament 10 into close engagement with the surface of the lead wire 12 due to the relative rigidity of the metal of the body 14. As a result the filament wire takes up a minimum of space in the region of the connection to the lead wire. This pressing operation may conveniently be carried out by forcing the tip member 18 against the ball or bead-like body 14 thereby to force the body 14 toward the lead wire.

In accordance with an important aspect of the present invention, there is created a firm and reliable connection between the body 14 and the filament 10 and also between the body 14 and the lead wire 12. As indicated in FIG. 3, this is accomplished by subjecting the body 14 to heat and to pressure to the end that the body 14 entraps or intimately envelops the filament 10 and at the same time forms an intermetallic or metallurgical bond with the lead wire 12.

More specifically, and in accordance with an important aspect of the invention, the body 14 of metal is heated to a temperature sufiicient to place the metal in a plastic or semi-plastic state, but insufficient to melt the metal. The temperature to which the body is heated in accordance with the invention is high enough so that the metal exhibits a substantial increase in intrinsic malleability and so that diffusion takes place at the interface of the body 14 and the surface of the lead wire 12. However, the temperature is maintained below the melting point of the metal of the body 14 and also well below the temperature at which the material of the lamp filament 10 becomes brittle.

In the preferred embodiment of the present invention, the body 14 is formed of gold and is heated to a temperature in excess of about 450 C., this being the approximate temperature at which gold is capable of diffusion. Furthermore the temperature of the body 14 is maintained below the melting point of gold which is about 1065 C. It has been found that satisfactory results are obtained by heating the gold body 14 to a temperature of approximately 600-650 C. for a period of about three seconds while pressure is applied to the body 14. This can conveniently be carried out by applying heat to the body 14 through the tip element 18, which may in turn be heated in any suitable manner.

During the operation of applying heat and pressure to the body 14, the metal of the body is in a highly malleable or semi-plastic state. As a result, under pressure the metal closely envelops or entraps the lamp filament 10 and an intimate, large area contact is created between the body 14 and the filament 10. Simultaneously, a thermocompression bond is created between the body 14 and the surface of the lead wire 12. More specifically, under conditions of heat and pressure diffusion of the materials at the joint interface of the body 14 and the lead wire 12 takes place to produce a strong and highly conductive intermetallic or metallurgical bond. As a result, the mechanical connection between the filament 10 and the lead wire 12 is firm and reliable. In addition, the electrical resistance at the interconnection is low.

One important advantage of the method of the present invention is that little or no oxide is produced during the operation of terminating the lamp filament. In some known methods of terminating a lamp filament metallic oxides may be produced due to the use of certain mate rials and/or as a result of the degree of heat required to complete the termination. When heated to elevated temperatures during subsequent use, such oxides may result in contamination of the vacuum within the lamp and/ or darkening of the transparent face plate of the lamp. Since the temperatures required in carrying out the process of the present invention are relatively low, and since the process makes use of gold in a pure form, the problems resulting from the production of oxides are substantially eliminated.

Having reference now to FIG. 4, after the bonding of the body 14 to the surface of the lead wire 12 together with the entrapping of the filament 10 has been completed, the tip member 18 is moved away from the body 14 in order to expose a segment of the metal wire 16. The wire is then severed above the body 14 in any suitable manner. In the preferred arrangement, the wire 16 is melted so that the severing of the wire and the formation of a new body of metal for a subsequent termination operation can be accomplished simultaneously. As illustrated in FIG. 4, a source of heat such as a flame torch 22 is applied to the wire 16 so that the wire melts and is severed. In addition, the melted metal at the end of the wire 16 is formed by the application of heat into a bead or ball serving as a metallic body such as the body 14 illustrated in FIG. 1.

When the wire .16 is severed from the body 14, a short length or trail of metal remains. As shown in FIG. 4, the trail may have a thin portion designated as 24 produced as a result of pulling the wire 16 while the metal on the body 14 remains in a plastic state. In order to reduce the profile of the connection between the filament 10 and the lead wire 12, the trail may be removed from the body 14 by breaking it off.

A significant advantage of the method of the present invention is that it may conveniently and economically be carried out with commercially available apparatus normally used for an unrelated purpose. For example, the invention may be carried out with thermocompression bonding equipment available from Wells Electronics, Inc. of South Bend, Ind. This equipment is ordinarily used for providing so-called nail head or ball bonds of a metal wire to a metal receiving surface such as a terminal of a microcircuit or the like, as described in an article entitled Microjoining Processes For Electronic Packaging, Part 3 by Ray B. Larson (Assembly Engineering, November 1966).

Although the present invention has been described with reference to details of one particular process embodying the invention, it should be understood that many alterations and modifications may be made by those skilled in the art without departing from the spirit and scope of the invention. Details of the process described here should not be taken to limit the invention except as set forth in the following claims.

What is claimed and desired to be secured by Letters Patent of the United States is:

1. A method of providing a mechanical and electrical connection between a tungsten lamp filament and a metal lead having at least a surface portion of gold comprising the steps of:

placing the filament on said surface portion of the lead;

placing a body of gold on the filament and over the surface portion of the lead;

heating the body of gold to a temperature below the melting point of gold and above the temperature at which the diffusion of gold takes place; and subjecting the body of gold to pressure of a magnitude great enough to cause the body of gold to surround the filament and to create a diffusion bond between the body of gold and the surface portion of the lead.

2. The method of claim 1, said step of heating, comprising heating said body of gold to a temperature in the approximate range of 600 to 650 C.

3. The method of claim 1 further comprising beading the end of a gold wire to form said body, and severing said wire from said body.

4. The method of claim 3, said severing step following said steps of heating and subjecting to pressure.

5. A method of terminating a spiral wound tungsten lamp filament to a lead wire having a gold surface comprising: advancing a gold Wire through a tip member; beading the end of said wire to form a body of gold; sandwiching the filament between said body and said surface; applying pressure to said body thereby to compress said spiral against said surface; heating said body simultaneously to entrap said filament in said body and to produce a diffusion bond between said surface and said body, and severing said Wire from said body.

6. The method of claim 5, further comprising continuing to apply pressure during said heating.

7. The method of claim 5, said step of applying pressure being carried out by applying force to said tip memher while said tip member is in engagement with said body, and said heating step being carried out by applying heat to said tip member while said tip member is in engagement with said body.

8. The method of claim 5, said heating step comprising heating said body to a temperature between about 450 C. and about 1065 C.

References Cited UNITED STATES PATENTS 1,238,575 8/1917 Schloter 29--25.13 2,082,079 6/1937 Palucki 29-25.1X 2,387,074 10/1945 Hroch 2925.1 3,221,386 12/1965 Demarest 2925.11 3,320,401 5/1967 Zachay et al. 228-44 JOHN F. CAMPBELL, Primary Examiner R. B. LAZARUS, Assistant Examiner US. Cl. X.R. 

